1. Field of the Invention
The present invention relates generally to an antenna, and more particularly to a tuning method of an antenna.
2. Description of the Prior Art
An antenna is a passive element used in a wireless communication device for transmitting and receiving electromagnetic signals. Compared with other passive elements, designing and developing an antenna is more complex because the antenna has a large number of target parameters should be taken into account during design and development, such as the dimensions, the resonant frequency, the bandwidth, the impedance matching and the gain, and so on. So far as mobile communication system and wireless local area network (WLAN) are concerned, though design of general compact-size antennas comes to mature, such as a planar inverted-F antenna, a microstrip antenna, a slot antenna, a patch antenna, etc, a problem of practical parameters being not up to the target parameters is still troubling. In conventional design, take a familiar planar inverted-F antenna (PIFA) for example, engineers usually calculate the theoretical dimensions of the PIFA at first, then make the antenna according to the theoretic dimensions, then tune the impedance matching by tuning the practical dimensions of the antenna, and finally tune the resonant frequency by tuning the practical dimensions of the antenna. When aiming at tuning the resonant frequency, a most common solution is to change the effective length of the radiating path which means the length between a feeder point of the antenna and an free end of a radiating element. For a PIFA, the radiating path should be a quarter of the operating wavelength of the antenna. The resonant frequency may be increasing by shortening the radiating path. Contrarily, the resonant frequency may be lowered down by lengthening the radiating path. Generally in making an antenna, the radiating path of the antenna is predetermined a little longer than the quarter of the operating wavelength at first, then gradually reduced to make a present resonant frequency gradually be close to the target resonant frequency with a best situation of the present resonant frequency equal to the target resonant frequency. However, once a tuning range of the length of the radiating path is excessive leading to the present resonant frequency overrunning the target resonant frequency, the above-mentioned solution will be invalid. In this case, a further remedial process is needed to tune a position of the feeder point more distant from the free end of the radiating element so as to increase the length of the radiating path. However, once the position of the feeder point is changed, the impedance will be mismatched which then needed to be tuned again and again. The whole tuning process is too time consuming.
Hence, in this art, an antenna with easily tuned resonant frequency and a method for easily tuning the resonant frequency of the antenna to overcome the above-mentioned disadvantages of the prior arts will be described in detail in the following embodiments.